The present invention relates to a method of controlling a fixing device, the fixing device and an image forming apparatus equipped with the fixing device.
In general, electro-photographic image forming apparatus is equipped with a hot-press type fixing device to fix toner images that are formed but unfixed on transfer sheets or recording materials. Among such hot-press type fixing devices, a well-known fixing device that provides a comparatively stable fixability (also referred to as fixing performance) comprises, for example, a heating roller containing a heat source such as a heater lamp, a supporting roller that is in parallel with the heating roller with a space between them, a fixing belt that is trained about the heating roller and the supporting roller, and a pressing roller that is placed to form a nipping area between the fixing belt and the pressing roller and gives a pressing force to the heating roller or the supporting roller via the fixing belt.
In the fixing device of this belt type, the pressing roller comprises a core bar made of metal such as aluminum and an elastic layer made of elastic material such as silicone rubber that covers the surface of the core bar.
This kind of fixing device is equipped with a temperature detecting means for detecting the temperature of the pressing roller. The fixing device sets the target temperature of the heating roller for optimum fixing according to temperature information sent from the temperature detecting means, causes a proper controlling mechanism to control turning on and off the heater lamp in the heating roller, and thus keeps the temperature of the nipping area at a preset temperature.
However, this controlling method that sets the target temperature of the heating roller from only temperature information sent from the temperature detecting means cannot keep the expected fixability (fixing performance) steadily as the hardness of the elastic material constituting the elastic layer around the surface of the pressing roller goes down when the fixing device is used for a long time or when the operating environment of the fixing device such as ambient temperature and humidity changes. Consequently, this kind of fixing device cannot be free from a problem that the formed images may have uneven glossiness.
Further, when a fixing condition such as the moving speed of the fixing belt or the fixing load changes, the fixing performance cannot be the same. Therefore, the fixing device cannot form images of even glossiness independently of the fixing process conditions.
Further, the conventional fixing devices have problems as explained below.
Glossiness of fixed images is one of the most significant image characteristics. As the glossiness of fixed images is dependent upon the fixing process conditions, the optimum fixing condition is set according to the expected glossiness of the fixed images. However, to change the glossiness of the fixed images, it is necessary to determine another optimum fixing process conditions by trial and error. This is a complicated and time-consuming work.
Even when an optimum fixing condition is set, for example, the hardness of the elastic material covering the pressing roller goes lower as the fixing device is used longer and the nipping width and the nipping surface pressure vary. In other words, the fixing process condition varies substantially. Therefore, it is difficult to form fixed images of the expected glossiness.
Further, the fixing device cannot be free from a problem that fixed images do not have uniform glossiness when the operating environment such as ambient temperature and humidity of the fixing device varies.
In view of the foregoing, a first object of the invention is to provide a method of controlling a fixing device to steadily retain a desired fixability (fixing performance) for a long time independently of a change or fluctuation in the fixing process condition or change in the operating condition.
A second object of the invention is to provide a fixing device that steadily retains a desired fixability (fixing performance) for a long time independently of a change or fluctuation in the fixing process condition or change in the operating condition.
A third object of the invention is to provide an image forming apparatus that steadily retains a desired fixability (fixing performance) for a long time independently of a change or fluctuation in the fixing process condition or change in the operating condition and steadily forms images of uniform glossiness.
A fourth object of the invention is to provide a method of controlling a fixing device that forms fixed images of a desired glossiness easily and steadily for a long time and that steadily forms fixed images of a desired glossiness independently of changes in the operating environment or the like.
A fifth object of the invention is to provide a fixing device that forms fixed images of a desired glossiness easily and steadily for a long time and that steadily forms fixed images of a desired glossiness independently of changes in the operating environment or the like.
A sixth object of the invention is to provide an image forming apparatus that forms fixed images of a desired glossiness easily and steadily for a long time and that steadily forms fixed images of a desired glossiness independently of changes in the operating environment or the like.
The above-described first through third objects can be accomplished by one of the structures below.
A method of controlling a fixing device comprising a heating roller containing a heat source, at least one supporting roller that is in parallel with the heating roller with a space between them, a fixing belt that is trained about the heating roller and the supporting roller, and a pressing roller that is placed to form a nipping area between the fixing belt and the pressing roller and gives a pressing force to the heating roller or the supporting roller via the fixing belt, wherein said method comprises the steps of (i) detecting the temperature of said pressing roller, (ii) calculating a target heating roller temperature (a) from a detected temperature (b) of the pressing roller obtained in said step (i), a loaded nipping time (c) calculated by Expression (1) below, a false nipping time (d) calculated by Expression (2) below, and a loaded nipping surface pressure (e) calculated by Expression (3) below, and (iii) controlling the operating status of the heat source in said heating roller from the target heating roller temperature (a) obtained in step (ii).
Loaded nipping time (c)=L1/Vxe2x80x83xe2x80x83Expression (1) 
False nipping time (d)=L2/Vxe2x80x83xe2x80x83Expression (2) 
xe2x80x83Loaded nipping surface pressure (e)=P/(L1xc3x97W)xe2x80x83xe2x80x83Expression (3)
In Expressions (1) to (3), L1 is the width of the fixing belt along the movement of the fixing belt in the surface area of the fixing belt sandwiched by the pressing roller and the supporting roller or the heating roller, L2 is the width of the fixing belt in the area in contact with the pressing roller only along the movement of the fixing belt, V is the moving speed of the fixing belt, W is the width of the fixing belt perpendicular to the movement of the fixing belt in the surface area of the fixing belt sandwiched by the pressing roller and the supporting roller or the heating roller, and P is the load (pressure) applied to the area sandwiched by the pressing roller and the supporting roller or the heating roller.
In the fixing device controlling method of the present invention, the target heating roller temperature (a) that satisfies Expression (4) below is preferably calculated in step (iii).
Axc3x97target heating roller temperature (a)+Bxc3x97detected pressure roller temperature (b)+Cxc3x97loaded nipping time (c)+Dxc3x97false nipping time (d)+Exc3x97loaded nipping surface pressure (e)+F=0,xe2x80x83xe2x80x83Expression (4) 
wherein A, B, C, D, E, and F are all constants.
Further in the fixing device controlling method of the present invention, it is preferable to detect the ambient temperature and humidity and change the above constants in Expression (4) according to the result of detection.
In addition, it is preferable to actually measure the L1 value in Expressions (1) and (3) and the L2 value in Expression (3) and calculate the loaded nipping time (c), the false nipping time (d), and the loaded nipping surface pressure (e) from the result of actual measurement.
The fixing device of the invention comprising a heating roller containing a heat source, at least one supporting roller that is in parallel with the heating roller with a space between them, a fixing belt that is trained about the heating roller and the supporting roller, a pressing roller that is placed to form a nipping area between the fixing belt and the pressing roller and gives a pressing force to the heating roller or the supporting roller via the fixing belt, a temperature detecting means to detect the temperature of the pressing roller, and a control mechanism to control the operating status of the heating source in the heating roller, wherein said control mechanism sets the target heating roller temperature (a) from a detected temperature (b) of the pressing roller obtained by said temperature detecting means, a loaded nipping time (c) calculated by Expression (1), a false nipping time (d) calculated by Expression (2), and a loaded nipping surface pressure (e) calculated by Expression (3), and controls the operating status of the heat source in said heating roller according to the target heating roller temperature (a).
In the fixing device of the invention, said control mechanism is preferably a means to set a target heating roller temperature (a) by Expression (4).
Further, it is preferable that the fixing device is equipped with an operating environment detecting means to detect the ambient temperature and humidity and that said control mechanism has a function to change constants in Expression (4) according to the ambient temperature and humidity detected by said operating environment detecting means.
Furthermore, it is preferable that said control mechanism has a function to receive at least the L1 value for Expressions (1) and (3) and the L2 value for Expression (2) from the outside and calculate the loaded nipping time (c), the false nipping time (d), and the loaded nipping surface pressure (e) from the L1 and L2 values that are entered from the outside.
The image forming apparatus of the invention is characterized by being equipped with said fixing device.
In accordance with the above structures, the target heating roller temperature (a) is set optimum corresponding to the actually detected pressure roller temperature (b) considering the loaded nipping time (c), the false nipping time (d), and the loaded nipping surface pressure (e). Therefore, the subsequent fixing process can be executed with the temperature of the nipping area optimized.
Accordingly, even when the fixing process conditions such as the nipping width change as the time goes by, the target heating roller temperature (a) is set according to the quantity of variation. This enables the fixing device to keep the desired fixing performance steadily for a long time independently of the change in the fixing process condition and operating environment.
The above-described fourth through sixth objects can be accomplished by one of the structures below.
A method of controlling a fixing device comprising a heating roller containing a heat source, at least one supporting roller that is in parallel with the heating roller with a space between them, a fixing belt that is trained about the heating roller and the supporting roller, and a pressing roller that is placed to form a nipping area between the fixing belt and the pressing roller and gives a pressing force to the heating roller or the supporting roller via the fixing belt, wherein said method comprises the steps of presetting a desired glossiness of fixed images, detecting the temperature of said pressing roller, calculating the target heating roller temperature (a) from at least the preset glossiness of fixed images, the detected temperature (b) of the pressing roller, a loaded nipping time (c) calculated by Expression (1) below, a false nipping time (d) calculated by Expression (2) below, and a loaded nipping surface pressure (e) calculated by Expression (3) below, and controlling the operating status of the heat source in said heating roller according to the calculated target heating roller temperature (a).
Loaded nipping time (c)=L1/Vxe2x80x83xe2x80x83Expression (1) 
False nipping time (d)=L2/Vxe2x80x83xe2x80x83Expression (2) 
Loaded nipping surface pressure (e)=P/(L1xc3x97W)xe2x80x83xe2x80x83Expression (3) 
In Expressions (1) to (3), L1 is the width of the fixing belt along the movement of the fixing belt in the surface area of the fixing belt sandwiched by the pressing roller and the supporting roller or the heating roller, L2 is the width of the fixing belt in the area in contact with the pressing roller only along the movement of the fixing belt, V is the moving speed of the fixing belt, W is the width of the fixing belt perpendicular to the movement of the fixing belt in the surface area of the fixing belt sandwiched by the pressing roller and the supporting roller or the heating roller, and P is the load (pressure) applied to the area sandwiched by the pressing roller and the supporting roller or the heating roller.
In the fixing device of the invention controlling method of the present invention, it is preferable to calculate the target heating roller temperature (a) that satisfies Expression (4xe2x80x2) below.
ln[G/(100xe2x88x92G)]=Axc3x97target heating roller temperature (a)+Bxc3x97detected pressure roller temperature (b)+Cxc3x97loaded nipping time (c)+Dxc3x97false nipping time (d)+Exc3x97loaded nipping surface pressure (e)+F,xe2x80x83xe2x80x83Expression (4xe2x80x2) 
wherein A, B, C, D, E, and F are all constants and G is the preset glossiness of fixed images.
Further in the fixing device controlling method of the present invention, it is preferable to detect the ambient temperature and humidity and change the above constants in Expression (4xe2x80x2) according to the result of detection. Further, it is preferable to actually measure the L1 value in Expressions (1) and (3) and the L2 value in Expression (3) and calculate the loaded nipping time (c), the false nipping time (d), and the loaded nipping surface pressure (e) from the result of actual measurement.
A fixing device comprising a heating roller containing a heat source, at least one supporting roller that is in parallel with the heating roller with a space between them, a fixing belt that is trained about the heating roller and the supporting roller, a pressing roller that is placed to form a nipping area between the fixing belt and the pressing roller and gives a pressing force to the heating roller or the supporting roller via the fixing belt, a temperature detecting means to detect the temperature of the pressing roller, and a control mechanism to control the operating status of the heating source in the heating roller,
wherein said control mechanism has a function for presetting the glossiness of fixed images to be formed, calculates the target heating roller temperature (a) at least from the preset fixed image glossiness, a detected temperature (b) of the pressing roller obtained by said temperature detecting means, a loaded nipping time (c) calculated by Expression (1), a false nipping time (d) calculated by Expression (2), and a loaded nipping surface pressure (e) calculated by Expression (3), and controls the operating status of the heat source in said heating roller according to the target heating roller temperature (a).
In the fixing device of the invention, said control mechanism is preferably a means to calculate the target heating roller temperature (a) by Expression (4xe2x80x2).
Further, it is preferable that the fixing device is equipped with an operating environment detecting means to detect the ambient temperature and humidity and that said control mechanism has a function to change constants in Expression (4xe2x80x2) according to the ambient temperature and humidity detected by said operating environment detecting means.
Furthermore, it is preferable that said control mechanism has a function to receive at least the L1 value for Expressions (1) and (3) and the L2 value for Expression (2) from the outside and calculate the loaded nipping time (c), the false nipping time (d), and the loaded nipping surface pressure (e) from the L1 and L2 values that are entered from the outside.
The image forming apparatus of the invention is characterized by being equipped with said fixing device.
In accordance with the above structures, as the fixing device is controlled according to a preset glossiness of fixed images considering factors that greatly affect the glossiness of fixed images, such as the heating roller temperature, the pressing roller temperature, the loaded nipping time (c), the false nipping time (d), and the loaded nipping surface pressure (e) that are calculated by Expressions (1) through (3), the fixing device can easily form fixed images of desired glossiness. Further, the fixing device calculates the target heating roller temperature (a) from the actually-detected pressing roller temperature (b) and controls the operating status of the heat source in the heating roller by the temperature (a) to keep the nipping area at an optimum temperature. With this, the fixing device can steadily form fixed images of desired glossiness.
When the fixing process condition such as a nipping width varies as the time goes by, the target heating roller temperature (a) is set according to the quantity of variation. With this, the fixing device can steadily form fixed images of desired glossiness for a long time.